Biometric authentication is a technology for identifying individuals based on biological features (palm veins, face, etc.) of human beings. In biometric authentication, first, feature data of the biological body of a person is registered as “registered biological body information”. At the time of authentication, feature data acquired with a sensor (such as a camera) is compared with the registered biological body information, the “similarity degree (degree of similarity)” expressing how much these data items are similar is calculated, and it is determined that the person being authenticated is the registered person if the similarity degree exceeds a predetermined threshold.
The feature data for biometric authentication is unlikely to be input based on exactly the same posture as that when the feature data had been registered. The posture refers to the positional relationship between a biological body (palm veins, face, etc.) and the sensor. Specifically, the posture is expressed by parameters such as a distance, a tilt angle, and a rotational angle.
A matching process is typically performed by allowing a predetermined variation in the posture. For example, image conversion such as parallel shift and rotation is performed on the input feature data, while comparing the feature data undergoing the image conversion with the registered biological body information, to search for the maximum similarity degree. In many cases, when the similarity degree exceeds a predetermined threshold, the matching process is ended. Accordingly, when there is a large variation between the posture at the time of registration and the posture at the time of authentication, excessive time is taken for the matching process because image conversion is performed. Furthermore, if the variation in the posture exceeds a predetermined range, it may not be possible to correct the feature data by image conversion, which deteriorates the precision in the authentication.
Accordingly, there is a disclosed technology for accumulating authentication results, determining whether the registered biological body information is to be registered again (reregistered) based on the frequency of unsuccessful authentication attempts, and prompting a user for which authentication has been frequently unsuccessful to reregister the registered biological body information. Furthermore, when the matching degree of the matching result is greater than or equal to an authentication determination threshold (90%) and less than or equal to a reregistration determination threshold (93%), it is determined that reregistration is to be performed, and the user is prompted to reregister the registered biological body information.
Furthermore, a technology has been developed for increasing the authentication precision by determining the posture at the time of authentication. For example, the following technology for increasing the precision of authenticating an individual is disclosed. In addition to performing biometric authentication with the master data used for authentication, a function/means for detecting differences in the posture may be provided. Accordingly, the posture may be corrected at the time of authentication.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-99374
Patent Document 2: Japanese Laid-Open Patent Publication No. 2008-102770
Patent Document 3: Japanese Laid-Open Patent Publication No. 2007-148724
As described above, in biometric authentication, it is determined whether a person being subjected to biometric authentication is the registered person by using registered biological body information as a reference. Therefore, the quality of the registered biological body information is significantly important. If the registered biological body information is inappropriate, the feature data may not be corrected even by applying various adjustments in the authentication process, and therefore an authentication error occurs.
Particularly, in the case of biometric authentication using images, the “posture” of the biological body with respect to the sensor has a large impact on the authentication precision. For example, in the case of “palm vein authentication”, (1) the position, (2) the distance, (3) the tilt angle, and (4) the rotational angle of the palm with respect to the sensor correspond to the posture (see FIG. 1).
FIG. 1 is for describing the position, the distance, the tilt angle, and the rotational angle of the palm with respect to the sensor in palm vein authentication. As illustrated in FIG. 1, the position, the distance, the tilt angle, and the rotational angle may be considered as parameters for expressing the “posture” of the palm. The following describes a case where images (biological body images) acquired by the sensor differ according to the posture, even though the vein patterns on the palm are the same.
FIG. 2(A) illustrates an example of a biological body in a normal position. FIG. 2(B) illustrates an example of a biological body in an abnormal position.
When a biological body image is acquired by a sensor, the biological body is photographed with the lighting on. The biological body image is affected by the distribution of the intensity of the lighting. The impact of the intensity distribution of the lighting is attributed to the fact that the intensity of the lighting is uneven within the field of view. Furthermore, in an optical system using lenses, limb darkening occurs, in which the apparent brightness decreases toward the periphery of the lens. The part denoted by “a” in FIG. 2(B) is near the periphery of the lens, and therefore the brightness is slightly lower. Thus, according to the position in the image, the apparent brightness changes (for example, see “a” in FIG. 2(B)), which has an adverse effect on the authentication process.
FIG. 3(A) is for describing the tilt angle of the palm with respect to the sensor. As illustrated in FIG. 3(A), when the palm is tilted, there is formed an area b that is close to the sensor and an area c that is away from the sensor. FIG. 3(B) illustrates an example of a biological body image obtained by photographing the palm in the posture illustrated in FIG. 3(A). Typically, in a photographed image, areas close to the sensor are bright and areas away from the sensor are dark. Therefore, in the example of FIG. 3(B), the area b appears to be bright and the area c appears to be dark in the biological body image. Accordingly, it is difficult to extract features in areas away from the sensor. Furthermore, it is difficult to extract features when the sensor is too close and overexposure is caused.
Furthermore, the photographed image is affected by lens distortion. Generally, the periphery of a lens is affected more by lens distortion than the center of the lens. Therefore, the posture of a subject largely affects the photographed image. As described above, the posture of a subject largely affects the similarity degree between the biological body information obtained by photographing the biological body and the registered biological body information.
In the conventional technology, in a case where the user is prompted to perform reregistration (to update the registered biological body information) when the similarity degree decreases, the following problem remains. That is, if the decrease in the similarity degree is attributed to unsteady postures of the subject, even if the information is updated, the precision in the similarity degree may not improve because the postures are still likely to be unsteady.
Furthermore, in the conventional technology, in a case where the posture at the time of registration is stored and the user is asked to correct his posture to the registered posture every time the authentication is performed, it may be inconvenient for the user.